It is known to coat zinc surfaces of galvanized metals with aqueous coating solutions that are effective in providing corrosion resistant coatings which protect the surfaces of the galvanized metals from corrosive degradation. In addition to serving to prevent or inhibit corrosion, such coatings also increase the adhesion properties of the surface to siccative coatings such as paints, lacquers and the like which may be subsequently applied to the metal for decorative or other purposes.
Generally, the compositions useful for this purpose are either acidic or alkaline. Alkaline coatings are widely used, examples of such coatings are disclosed in U.S. Pat. Nos. 3,444,007; 3,515,600 and 4,278,477. Acidic compositions which form phosphate or chromate coatings on zinc, are described in U.S. Pat. No. 3,297,494. The fluorides in these coating compositions are usually in the form of complex fluorides such as H.sub.2 ZrF.sub.6, H.sub.2 TiF.sub.6, and H.sub.2 SiF.sub.6. Such acidic coating compositions have been tried on galvanized metals but did not always function as desired. In addition, the dissolution of zinc from the surface of the metal being treated resulted in zinc contamination of the treatment bath and of the overflow from the treatment tanks. Because of the sludge producing nature of zinc solutions, the treatment tanks were overflowed in order to maintain relatively low concentrations of zinc in the treatment tanks. The presence of zinc in the overflow has given rise to concerns regarding the safe and environmentally acceptable treatment or disposal of the zinc contaminated overflow.
Patent Cooperation Treaty International Publication Number WO 85/05131 discloses an acidic aqueous coating solution to be applied to galvanized metals to increase their resistance to corrosion which contains from 0.1 to 10 grams per liter, based on fluoride content, of a fluoride-containing compound, and from 0.015 to 6 grams per liter, based on metal content, of a salt of cobalt, copper, iron, manganese, nickel, strontium, or zinc. Optionally, a sequestrant and/or a polymer of a (meth)acrylic acid or ester thereof can also be present.